Cyanides are known to be harmful to our environment and toxic to man and more so to aquatic life. The cyanide compounds are produced and introduced to our environment in many damaging ways. Free cyanide is a highly toxic chemical that is found in the environment at low concentrations coming from natural sources. It reaches toxic levels mostly through industrial processes such as mineral processing, electroplating, and papermaking. The Resource Conservation and Recovery Act ban on land disposal of solid waste containing cyanides poses a major waste management problem for industries using cyanide in their processes.
Cyanide is also a regulatory target because of it toxicity, incompatibility with most publicly owned treatment plants, and danger to sewer workers and marine life. The EPA has imposed limits on the quantity of cyanide in both the treated wastewater that is discharged to sewers and rivers and on any residuals from metal finishing operations (sludges, filters, filter cakes, spent solutions, etc.). A variety of electroplating and metal finishing waste streams contain metal-cyanide complexes. Metal cyanide complexes formed in these industries include metals such as iron, nickel, zinc, cobalt, cadmium, copper, mercury, and precious metals (silver, gold, and platinum).
There is some free cyanide found in electroplating wastewater from cyanide-based plating chemistries; it is one of the most toxic contaminants in the wastewater. Evaporation, while effective, has high energy costs. The commonly used cyanide destruction techniques, such as treatment with oxidizers, do not easily destroy all the cyanide. For example, iron, cobalt, and nickel cyanides are not affected by basic hypochlorite treatment and are often precipitated out into sludge that is formed under the process. Thus, elevated levels of complexed cyanide typically appear in hydroxide-precipitated, heavy metal sludges produced during the treatment of many electroplating wastewater solutions. Waste and wastewater having highly toxic forms of cyanide should be detoxified to a level acceptable to the environment.
Speciation and measurement of the different forms of cyanides are very important to understand the levels to toxicity of each species and offer important steps to protect and improve our environment. Of the various analytical methodology and equipment employed to measure cyanides, automated equipment have achieved more notable results. For example, refer to U.S. Pat. No. 4,265,857 "In-Line Distillation System" and U.S. Pat. No. 4,804,631 "Method and Apparatus for Measuring Cyanide", and to a publication entitled "Chemistry of Wastewater Technology" published by Ann Arbor Science Publishers Inc., of Ann Arbor, Mich., Library of Congress Catalog Card No. 76-50991, ISBN0250, 40185-1, and to a paper by N. P. Kelada entitled "Automated Direct Measurements of Total Cyanide Species and Thiocyanate, and Their Distribution in Wastewater and Sludge", published in the Journal of the Water Pollution Control Federation, Volume 61, No. 3, March 1989, which are incorporated herein by reference.
As described in Chapter 2 of "Chemistry of Wastewater Technology" and in the paper of WPCF Journal such an automated system involves the steps of separation, absorption, and measurement, with ultraviolet irradiation being employed to dissociate complex cyanides in the process of separation, along with thin film distillation and chemical absorption techniques.
A conventional UV irradiation unit includes a UV mercury lamp surrounded by a quartz coil, through which the sample to be tested is passed. The quartz coil is permeable to substantially all of the UV spectrum (approximately 150 to 400 nanometers) enabling the UV radiation to break down the cyanide strong complexes including iron and cobalt cyano complexes. The cobalt very strong complex absorption bands are around 255 and 310 nanometers. However, unwanted thiocyanate (SCN) is also dissociated and detected along with the cyanides.
In advanced UV irradiation units a filtering device is interposed between the UV lamp and the sample to be tested, for passing only the lower frequency UV radiation (longer than 290 nm) for breaking cyanide complexes, while blocking high frequency to inhibit the dissociation of thiocyanate. The "in-line distillation system", or rather thin film distillation, separates the resulting hydrogen cyanide from the acidified sample, and the HCN gas is then absorbed in a sodium hydroxide and recovered for subsequent calorimetric measurement. Thus all of the cyanides can be detected and measured in a large number of samples using an automated segmented flow system.
The above cyanide measuring system, though acceptable for several applications, is cumbersome and has some complicated modules for irradiation separate from the fragile thin film distillation. Additionally, excessive heating and samples containing high gas content could interrupt the complicated waste system, which disrupt the continuous operation causing the loss of many samples and results. In addition, this thin film distillation use is limited to segmented flow systems. More importantly, the previous system does not distinguish all the cyanide species, in particular the dissociable cyanide and the complex cyanide, and is liable to many interferences.
It would be highly desirable to have an improved analytical system that can distinguish between the different cyanide species and can directly measure the dissociable cyanide separately from the total cyanides. It would be beneficial to have a simpler, more rigid apparatus combining both irradiation and distillation in one unit that would not be liable to interruption by high gaseous content of samples. It would also be advantageous if the new technique could operate with either segmented flow or flow injection systems.